HIBEAM/NNBAR annihilation detector development
by
A5:3007
AlbaNova Main Building
Abstract:
The observed matter-antimatter asymmetry in the universe implies a fundamental creation of matter over antimatter, a process linked to baryon number violation. Yet, experimental evidence for such a violation remains elusive. The HIBEAM/NNBAR programme, proposed at the European Spallation Source, seeks to probe baryon
number violation through a comprehensive two-stage experiment. This includes investigating neutronantineutron oscillation, neutron-antineutron regeneration from a sterile state, and neutron disappearance and regeneration. The program’s objective is to enhance the sensitivity of neutron conversion probability measurements by up to three orders of magnitude beyond previous efforts, potentially unravelling key mysteries
in baryogenesis, dark matter, and new physics beyond the current collider research.
This thesis focuses on the development of the design of the annihilation detector for the HIBEAM/NNBAR programme, with a particular emphasis on the NNBAR annihilation detector. It investigates the multiplepion state resulting from neutron-antineutron annihilations, which approximately release energy equivalent to two neutron masses. The detector’s design integrates various technologies, including a Time Projection Chamber, plastic scintillators for hadronic range calorimetry, and lead glass for Electromagnetic calorimetry. A detailed GEANT4-based simulation study is conducted to analyze various event types, including annihilation, cosmic ray background, and neutron beam background. The study also involves examining the unique signatures these
events leave in the detector and systematically refining the detector’s optimization.
This thesis delves into the techniques of object reconstruction, which involves combining data from various components of the GEANT4-based detector simulation. The crucial task in this process is the identification of these reconstructed objects. A detailed discussion and computation of event variables, derived from the detector information, are presented. These variables are instrumental in differentiating between annihilation signal events and cosmic ray background events. A preliminary event selection scheme is introduced, based on these variables, to differentiate between annihilation signal events and cosmic ray backgrounds. Preliminary results, with limited statistical data, indicate that this cut-based selection scheme can achieve approximately 70% efficiency in signal acceptance while completely rejecting cosmic ray background.
